Sudden cardiac arrest is commonly treated mechanically and/or by electrical defibrillation. Mechanical treatment may be given manually or by a chest compression apparatus. A number of chest compression apparatus are known in the art, such as the pneumatically driven LUCAS™ mechanical chest compression system (“Lucas™ system”; an apparatus for compression and physiological in CardioPulmonary Resuscitation, CPR, manufactured by Jolife AB, Lund, Sweden). Specifically the Lucas™ system comprises a support structure and a compression unit. The support structure includes a back plate for positioning under the patient's back posterior to the patient's heart and a front part for positioning around the patient's chest anterior to the heart. The front part has two legs, each having a first end pivotally connected to a hinge of the front part and a second end removably attachable to the back plate. The front part is devised to centrally receive the compression unit, which is arranged to repeatedly compress the patient's chest. The compression unit comprises a pneumatic means arranged to drive and control compression, an adjustable suspension means to which a compression pad is attached, and a means for controlling the position of the pad in respect of the patient's chest. The use of a pneumatic means as the driving force relies on a reciprocating piston providing compressions on the chest by the pad, driven by pressurized gas. The system utilizes pressurized gas for driving the piston both ways, i.e. in the direction of the patient's chest (compression phase, gas being supplied to a compression chamber) and then in the opposite direction (gas being supplied to a decompression chamber), whereby the sternal portion of the chest is brought back to its original position (decompression phase). The consumption of pressurized gas can be substantial and is a limiting feature on the use of the apparatus in places where supply of pressurized driving gas is limited. The consecutive supply of driving gas to the two chambers of the known apparatus requires a complex and thus expensive valve system and a correspondingly complex control.